It is desirable to minimize the amount of human labor expended in maintaining and cleaning buildings. The art has therefore developed autonomous robotic devices that can clean or otherwise maintain or treat hard floors, carpeting and similar surfaces without the necessity for a human to be present during the operation of the device.
In some such devices a liquid is applied to the flooring area being treated. For example, U.S. Pat. No. 5,279,672 discloses a robotic cleaning apparatus where a cleaning solution is dispensed to the floor by a scrub deck. In U.S. Pat. No. 6,741,054 there is disclosed an autonomous floor mopping apparatus where cleaning fluid is applied to the floor by way of a pre-moistened towel.
Such robotic devices typically have a programmable controller for directing the device in a preferred movement pattern. This helps insure coverage of the full area to be treated, as well as helping to insure that obstacles (e.g. furniture legs) and undesired contact points (e.g. stairways) are avoided. The controllers are typically linked to motors that drive the wheels of the device on the floor.
While devices of this type can work quite well on dry surfaces, the wheels of such devices may slip when traveling over areas of the floor that are wet from the fluid being applied. This is particularly likely when the liquid itself is of the type which, when wet, is significantly more slippery than water (e.g. contains oil for polishing purposes). Such slipping can cause the device to remain in place for an extended period, or more likely cause the device to divert in an unexpected direction from the optimal desired path. This can extend the time needed to treat the surface, and/or can lead to portions of the surface not being adequately treated.
In some robotic devices, a third wheel that is not driven by a motor is used to monitor the movement of the robot. This third wheel has an optical or mechanical sensor (encoder) that will send a digital signal to the controller as long as the robot is moving. Hence, if the robot is in a moving mode, but this third wheel does not sense movement, then the controller knows it is slipping. This method detects slip. This third wheel may be called a stator wheel.
In connection with automobile and truck tires there has been substantial work on trying to improve the traction of the tires through the use of varied tread patterns. However, many of these approaches are designed to take advantage of the very heavy weight of such vehicles, and are not easily transferred to environments where a cleaning robot is involved that weighs much less. Others of these approaches rely on expensive materials, or structures that are relatively expensive to create.
Similarly, in connection with automobiles and trucks, there have been attempts to provide improved anti-slip control by monitoring wheel movement and automatically altering power to the wheels when sensing such slip. Because such controller systems were designed for extremely heavy vehicles, they were not easily transferred to environments where a cleaning robot was involved that weighed much less. Further, some systems that could be transferred to a small cleaning robot were of too great a cost to be used in that environment as a practical matter.
Hence, a need still exists for improved structures and systems for addressing wheel slip concerns in the context of an autonomous floor cleaner.